Context. The changing-look phenomenon observed in a growing number of active galaxies challenges our understanding of the accretion process close to a black hole. Aims. We propose a simple explanation for the sources where multiple semi-periodic outbursts are observed, and the sources are operating close to the Eddington limit. Methods. The outburst are caused by the radiation pressure instability operating in the narrow ring between the standard gas-dominated outer disk and the hot optically thin inner advection-dominated accretion flow. The corresponding limit cycle is responsible for periodic outbursts, and the timescales are much shorter than the standard viscous timescale due to the narrowness of the unstable radial zone. Results. Our toy model gives quantitative predictions and works well for multiple outbursts like those observed in NGC 1566, NGC 4151, NGC 5548, and GSN 069, although the shapes of the outbursts are not yet well modeled, and further development of the model is necessary.

We study the long-term variability in the optical monitoring database of Ark 120, a nearby radio-quiet active galactic nucleus (AGN) at a distance of 143 Mpc (z = 0.03271). We compiled the historical archival photometric and spectroscopic data since 1974 and conducted a new two-year monitoring campaign in 2015–2017, resulting in a total temporal baseline over four decades. The long-term variations in the optical continuum exhibit a wave-like pattern and the Hβ integrated flux series varies with a similar behavior. The broad Hβ profiles have asymmetric double peaks, which change strongly with time and tend to merge into a single peak during some epochs. The period in the optical continuum determined from various period-search methods is about 20 yr, and the estimated false alarm probability with null hypothesis simulations is about 1 × 10⁻³. The overall variations of the broad Hβ profiles also follow the same period. However, the present database only covers two cycles of the suggested period, which strongly encourages continued monitoring to track more cycles and confirm the periodicity. Nevertheless, in light of the possible periodicity and the complicated Hβ profile, Ark 120 is one candidate of the nearest radio-quiet AGNs with possible periodic variability, and it is thereby a potential candidate host for a sub-parsec supermassive black hole binary.

The identification of a virial broadening estimator in the quasar UV rest frame suitable for black hole mass computation at high redshift has become an important issue. We compare the H i Balmer H β line width to the ones of two intermediate-ionization lines: the Al iii λ1860 doublet and the C iii] λ1909 line, over a wide interval of redshift and luminosity (0 ≲ z ≲ 3.5; 43 ≲ log L ≲ 48.5 [erg s⁻¹]), for 48 sources belonging to the quasar population characterized by intermediate to high values of the Eddington ratio (Population A). The present analysis indicates that the line widths of Al iii λ1860 and H β are highly correlated and can be considered equivalent for most Population A quasars over five orders of magnitude in luminosity; for C iii] λ1909, multiplication by a constant correction factor ξ ≈ 1.25 is sufficient to bring the FWHM of C iii] in agreement with that of H β. The statistical concordance between low-ionization and intermediate-ionization lines suggests that they predominantly arise from the same virialized part of the broad-line region. However, blueshifts of modest amplitude (few hundred kilometers per second) with respect to the quasar rest frame and an excess (≲1.1) Al iii broadening with respect to H β are found in a fraction of our sample. Scaling laws to estimate M _BH of high-redshift quasars using the Al iii and C iii] line widths have rms scatter ≈0.3 dex. The Al iii scaling law takes the form log M BH ≈ 0.58 log L 1700,44 + 2 logFWHM + 0.49 [M _⊙].

The 4D (four-dimensional) eigenvector 1 (E1) sequence has proven to be a highly effective tool for organizing observational and physical properties of type-1 active galactic nuclei (AGNs). In this paper, we present multiple measurements of metallicity for the broad line region gas, from new and previously-published data. We demonstrate a consistent trend along the optical plane of the E1 (also known as the quasar main sequence), defined by the line width of Balmer hydrogen H$\beta$ profile and by a parameter measuring the prominence of singly-ionized iron emission. The trend involves an increase from sub-solar metallicity in correspondence with extreme Population B (weak Feii emission, large H$\beta$ FWHM (full width at half maximum)) to metallicity several tens the solar value in correspondence with extreme Population A (strongest Feii optical emission, narrower H$\beta$ profiles). The data establish the metallicity as a correlate of the 4DE1/main sequence. If the considerably high metallicity ($Z\gtrsim 10Z_{\odot }$, solar metallicity) gas is expelled from the sphere of influence of the central black hole, as indicated by the widespread evidence of nuclear outflows and disk wind in the case of sources radiating at a high Eddington ratio, then it is possible that the outflows from quasars played a role in chemically enriching the host galaxy.

Quasars accreting matter at very high rates (known as extreme Population A (xA) or super-Eddington accreting massive black holes) provide a new class of distance indicators covering cosmic epochs from the present-day Universe up to less than 1 Gyr from the Big Bang. The very high accretion rate makes it possible that massive black holes hosted in xA quasars can radiate at a stable, extreme luminosity-to-mass ratio. This in turn translates into stable physical and dynamical conditions of the mildly ionized gas in the quasar low-ionization line emitting region. In this contribution, we analyze the main optical and UV spectral properties of extreme Population A quasars that make them easily identifiable in large spectroscopic surveys at low- ( $z\lesssim 1$ ) and intermediate-z (2 $\lesssim z\lesssim$ 2.6), and the physical conditions that are derived for the formation of their emission lines. Ultimately, the analysis supports the possibility of identifying a virial broadening estimator from low-ionization line widths, and the conceptual validity of the redshift-independent luminosity estimates based on virial broadening for a known luminosity-to-mass ratio.

We analyzed the light curves (LCs) of several radio-quiet and radio-loud quasars belonging to the same parameter space volume in the 4D Eigenvector 1 (4DE1) quasar sequence, using data from the Catalina Real-time Transient Survey (CRTS). We report preliminary results on detected variability pattern, and discuss possible cases of periodic variability.